1. Field of the Invention
The present invention relates to a wire bonding method, by which pads of a semiconductor chip and inner leads are connected with bonding wires.
2. Description of the Prior Art
In semiconductor devices, wire bonding is conducted to connect a pad as an electrode of a semiconductor chip with an inner lead such as a lead frame, using a bonding wire such as a gold wire. A conventional apparatus for such wire bonding has a capillary for moving a bonding wire between a pad to be bonded and an inner lead to be bonded second and pressing the bonding wire against the pad and the inner lead, and has a torch for forming a ball on an end of the bonding wire projecting from the tip of the capillary (as in Japanese Patent Laid-Open No. 124435/1988).
On the other hand, the wire bonding can be specifically explained by the following steps as shown in FIGS. 3 and 4. First, a torch forms a ball 31 on an end of a bonding wire 30 (see FIG. 3(a)). A capillary 40 crushes the ball 31 by pressing to it to a pad 11. After the ball 31 is crushed, ultrasonic waves at a constant output are applied to the ball 31 during a period from a time T.sub.2 to a time T.sub.4 (FIG. 4) with the capillary 40 pressing the ball 31 against the pad 11, so that the ball 31 and the pad 11 are bonded together (see FIG. 3(b)). When the ball 31 is completely bonded to the pad 11, the capillary 40 is moved to a position for bonding an inner lead, where a second bonding is performed. (See FIG. 3(c). The capillary 40 leaves the ball 31 at a time T.sub.5 (FIG. 4.). In addition the capillary 40 is vertically moved by a pulse motor or the like.
In the above mentioned wire bonding, the load required for pressing of the ball 31 against the pad may be varied before and after the pressing, as disclosed in Japanese Patent Laid-Open Nos. 101039/1986, 208836/1986 and 41217/1988.
However, such a conventional wire bonding method has disadvantages. For example, when the pad 11 is thin and a metal forming the pad 11 has insufficient hardness, the ball 31 gets into the pad 11 in pressing the ball 31 to the pad 11, and sometimes the ball 31 approaches an insulation oxide film 12 just below the pad 11 as shown in FIG. 5. Since the insulation oxide film 12 has a low mechanical strength, the insulation oxide film 12, a Locos film 14 and the like just below the pad 11 are damaged as, for example, cracked (See 121 of FIG. 5) when strong ultrasonic waves at a constant output are applied under these conditions. This is particularly likely, when a material having a hardness higher than that of gold, such as copper and silver, is used for bonding wire, as the ball more easily gets into the pad. As a result, the interval or spacing between the ball and the insulation oxide film becomes narrow, and ultrasonic waves are directly applied to the insulation oxide film. This causes more serious damage.